Power converters are used to convert power from an input bus connected to a power source for a load. Power converters may be designed to convert a fixed AC input voltage into a variable voltage and/or variable frequency output. Other converters are designed to convert an AC input to DC. Still others convert a DC input at one voltage to another voltage. The architecture chosen may provide for high frequency operation, pulse-width-modulation, transformer isolation, and the like. Many power converters include a number of cells also called blocks or modules.
Depending on input and output voltage/current requirements (high voltage, low current or low voltage, high current), the input and output sides of the converter cells can be connected in series or parallel.
In general, four combinations of the cell input/output connection are possible: Input Series Output Series (ISOS), Input Series Output Parallel (ISOP), Input Parallel Output Parallel (IPOP), and Input Parallel Output Series (IPOS). Among those four, ISOP and IPOS architectures are more commonly adopted, since they provide inherent voltage and current sharing between the cells.
It should be noted that three out of four architectures employ series cell connection. Since series-connected cells share the same current, fault isolation in case of a single or multiple cell failure is difficult. Typically, converter operation has to be interrupted to disconnect and/or replace failed cell(s).
Also, in case of a cell failure, the remaining cells have to sustain higher operating voltage and/or current. While the high current condition can be mitigated by shedding non-critical loads, reducing input and output voltage is usually not possible.
The mechanical arrangement of high voltage converter assemblies with replaceable cells presents additional challenges particularly in applications with limited space. One prior design includes cells with grounded cases connected in series across the input bus. While this approach simplifies multi-module construction and improves assembly flexibility, each cell has to sustain the full bus voltage from the components to the chassis resulting in an increase in size and weight.
Power converter equipment relies on various methods of grounding to ensure safety and to control transient processes resulting from ground faults. One prior approach describes multiple identical modules series-connected across the input and a direct connection between the input return of the high voltage stack and chassis ground. While this technique accords simple fault detection and isolation since a ground fault becomes equivalent to a short circuit, it increases current stress for the semiconductors used in the cells. The housings of all the cells are grounded to allow simple installation and replacement, but the cell has to sustain the full working voltage from the components to the chassis.
An alternative technique uses grounding of the center point of the cell stack through a high resistance to ground to maintain the center point at zero potential under normal conditions. This method reduces the insulation requirements for the input components thereby allowing more efficient use of space but it does not provide the optimal solution because all of the components on the secondary side have to be insulated from the chassis to sustain 50% of the working voltage.